Lots of people keep their cat inside year round. Animal welfare advice is that this is not cruel to cats. However, cats are territorial and keeping a cat which has an established outside territory locked up and unable to patrol its territory will cause some distress which is unfair. Indoor cats do, of course, need access to food, water and a regular cleaned out litter tray. Try and keep the litter tray in a private place. Cats are used to finding food in varying locations and you can encourage them to drink more if you move their water bowl around frequently. They will need something to scratch and since you don't want this to be your soft furnishings install a scratching post. Cats do need access to vegetation so grow some grass for them in a pot. Eating grass and other vegetation is thought to aid digestion and some pet shops sell ready sown grass you can grow yourself precisely for this purpose. Younger cats will want to play so make sure you give them items to chase. Outside they will chase tumbling leaves and bat the ripples in a pond as well as stalking and killing prey. Make sure your indoor cat is similarly stimulated.